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Good yet inexpesnive material for CNC mold/tool

I've been playing around with the idea of building my own composite boat and I have a 3D model drawn up that I want to work out on a CNC mill. I just wanted to get some opinions/advice from experts about what material would be a good yet inexpensive alternative to bondo? I need about 23 gallons to pour the entire mold that will then be cut out on the CNC. Thing is, at nearly $30 a gallon for the bondo I'm wondering if plaster or even layered MDF panels might still work. Is there something even better/cheaper I haven't heard of for this purpose? I've used bondo for smaller molds, but for the shear size of this I was hoping to cut some costs.Thanks in advance

RE: Good yet inexpesnive material for CNC mold/tool

MDF works if you cut it, then soak it in resin. then cut it again and repeat a few times. the resin causes the mdf to swell, which is the reason for having to re-cut it. You can then sand and polish the part surface.

RE: Good yet inexpesnive material for CNC mold/tool

I'm not sure how much this stuff costs but I've heard of it being used for doing just what you say you want. You might want to look into it or at least check the internet for foams used in CNC prototyping.

RE: Good yet inexpesnive material for CNC mold/tool

I was thinking I might have to do something like that with MDF, although I haven't used it for this purpose.Â*

I just had a thought, might I be able to find a medium to high density foam somewhere, like maybe exterior sheething (the fiberglass stuff, not the styrofoam), then treat it with resin or something similar to seal the surface?

RE: Good yet inexpesnive material for CNC mold/tool

If you want inexpensive and fast; get yourself some good old 1 or 2 lb density Styrofoam, it can be purchased in thick blocks when purchased from a foam fabricators supply houses. Fasten foam to a base that is repeatably indexed as you would want to do anyway and cut your shape .050" smaller then your finish pass, next apply a wet layup with glass and epoxy resin equaling around .080" thick to the surface. NOTE: this only around 4 plies of style 7500 tooling cloth. Place back on the machine and re-cut to your finish dimension, prime fill surface, release and go. You can do this same operation with MDF, and both will yield a stable surface, the Styrofoam is just a little cheaper to purchase and allows you to cut a little faster if that is important to you. I manufacture thousands of MMTs (Master Model Tools) each year this way along with REN tooling board MMTs, it usually all comes down to money. Anyway best of luck to you. Here are a couple of pictures of an MDF and REN board engine nacelle MMT being cut undersized, glassed and re-cut to final dimensions for stability back in 2008.

RE: Good yet inexpesnive material for CNC mold/tool

We have just done something similar to what sensei reported.
Machined a plug out of blue Styrofoam, then surfaced it with glass cloth. Not finished machined, just filled and sanded. The moulds were then made from the plugs. One of the plugs was machined in several parts which was then assembled into one whole for making the plug.

RE: Good yet inexpesnive material for CNC mold/tool

We get our blue Styrofoam from a building supply or lumber store. Pink foam would work as well. Both have about a 2 lb density and uniform structure, not like the white foam. I live north, so it is used a lot for home insulation.

RE: Good yet inexpesnive material for CNC mold/tool

we make prototype vaccum form molds from MDF. After rough machining they get a coat of poly resin then re machined. The poly resin is thinned pretty well so that it soaks in well. If you are going to use it for a layup mold, I would finish it very well with a quality epoxy resin and catalyzed paint.